The present invention relates to a method for producing holograms. The invention is particularly useful for producing holographic optical elements (HOEs) for helmet displays, and is therefore described below with respect to this application.
Holographic optical elements (HOEs) have several advantages compared to conventional elements. They are lighter, more compact, and can be formed and replicated with relative ease. Even more important is the fact that they can sometimes perform complex optical operations which cannot be achieved with conventional elements. In general, the HOEs must be used with a monochromatic light source.
An important application for such HOEs is to serve as an imaging lens and combiner for a helmet mounted display. Here, a two dimensional monochromatic display is imaged to infinity and is reflected to a pilot's eye. The display can be direct from a CRT, or indirect through a relay lens or an optical fiber bundle. The display is comprised of an array of points whose geometrical conditions at read out will differ from those at recording. Consequently, these points will contain aberrations that decrease the quality of the image. In order to minimize these aberrations, it is necessary to include corrective elements, e.g., relay lenses, which are particularly disadvantageous in helmet displays because of the substantial increase in weight caused by the addition of such corrective elements.
Recently there have been several proposals for designing imaging holographic lenses with improved performance. In these designs, aspherical, rather than simple spherical waves, are used for recording the HOEs. The aspherical waves were derived from conventional optics, or from computer-generated-holograms (CGH). Unfortunately, such approaches for obtaining the aspherical waves must rely on fairly complicated and costly components and equipment. Moreover, the aberrations are not completely corrected during recording, so that some corrective elements are necessary for read out; as a result the overall helmet display unit becomes more complex, cumbersome, and heavy.
An object of the present invention is to provide a new method for producing desired holograms having improved performance and particularly useful for making holographic optical elements for helmet displays.